Most transistor amplifiers are either class A, AB or B. the output transistors are usually bipolar transistors that are operated in their linear region. Current limit is implemented using sense resistors in series with the output transistors to develop a voltage proportional to the output current. When this voltage is too high, the base current of the output transistors is limited, thus limiting the output current to a safe level. Linear amplifiers using MOSFETs employ similar circuits to limit the gate voltage on the output MOSFETs in order to limit the output current. These approaches do not work in class D amplifiers because the MOSFETs are either off or fully enhanced. The traditional solution is too offer over current latch off as opposed to current limit. In that solution sense resistors are placed in series with the MOSFETs. When the voltage across the sense resistors is too high, the amplifier shuts down. The drawback to that approach is that the impedance of speakers varies with frequency, and it may dip low enough at some frequencies to shut down the amplifier.
It is possible to regulate the output current with a separate control loop, which is independent of the audio loop. The goal of the second loop is to override the main control loop when the current is excessive. The disadvantage of this approach is that it is expensive and relatively difficult to stabilize due to the delays associated with filtering the voltage across the sense resistor to remove the noise associated with switching transitions.